A considerable body of experimental evidence has implicated an important role for the cellular ras oncogenes in human carcinogenesis. Recently, the human Krev-1 gene has been shown to suppress the transforming activity of oncogenic ras. Surprisingly, the Krev-1 protein shares significant sequence homology (50%) with human ras proteins. Thus, while Krev-1 and ras share significant structural and biochemical properties, differences must exist to account for their divergent biological activities. The biochemical basis for Krev-1 suppressing activity is not known. The overall goal of this proposal is to characterize the structural and biochemical properties of the Krev-1 protein, and to determine which properties are important for Krev-1 suppression of oncogenic ras activity. The approaches outlined in this proposal are based on information derived from structure-function studies of the ras oncogene proteins. For these studies, structural mutations have been introduced into Krev-1 sequences corresponding to functional domains in ras to delineate the biochemical similarities and differences between these two proteins. Additionally, chimeric proteins have been generated between Krev-1 and H-ras to identify the Krev-1 domains responsible for its dominant suppressing activity. Furthermore, since there are strong structural and functional similarities between Krev-1 and oncogenic ras, the possibility that Krev-1 is a proto- oncogene protein will also be evaluated. Finally, since oncogenic ras is frequently associated with human carcinomas, the ability of Krev-1 to antagonize ras transforming activity in cells of epithelial origin will be evaluated. Although the mechanism of ras suppression by Krev-1 is not known, the strong structural similarities with ras proteins suggest the possibility that Krev-1 may antagonize the activity of oncogenic ras by competing for common target or regulatory proteins. Alternatively, Krev-1 may modulate a negative growth regulatory pathway which indirectly antagonizes the positive growth regulatory pathway modulated by oncogenic ras. The studies outlined in this proposal will generate important information for distinguishing between these two possibilities and provide a biochemical basis for the Krev-1 suppression of oncogenic ras activity.